Nanometer-scaled composites provide the opportunity to combine useful attributes of two or more materials within a single composite or to generate entirely new properties as a result of the intermixing of the two components. Many useful biological composite materials, for example, can be found in Nature, including mollusk shells, teeth and bone. For instance, see J. D. Currey, J. Mater. Educ., 9, 120 (1987) and H. A. Lowenstam and S. Weiner, On Biomineralization, Oxford University Press, New York, 1989.
Mollusk shells, consist of highly organized laminated microstructures of aragonite CaCO3 crystals (with thickness of ˜250 nm) separated by a thin (30-50 nm) layer of proteinaceous organic matter. The resulting strength and toughness of this brick and mortar structure is orders of magnitude higher than those for either of the constituents alone. The useful properties of this biological composite arise from the highly organized and appropriately proportioned combination of a hard brittle inorganic phase with a soft plastic organic phase, with strong interfacial bonding between the two components.
A second example, this time of a synthetic nanocomposite, can be found in polyimide chemistry. Polyimides are used for microelectronic applications because of their heat resistance, chemical stability, and superior electrical properties. In an effort to reduce the coefficient of thermal expansion and the amount of moisture absorption, a small amount of clay (montmorillonite) is dispersed on a molecular level in the polymer. Only about 2 weight percent addition of montmorillonite lowers the permeability coefficients for various gases to less than one-half of the values for pure polyimide, while at the same time reducing the thermal expansivity. For example, see K. Yano, A. Usuki, A. Okada, T. Kurauchi, O. Kamigaito, J. Polym. Sci.: Part A: Polym. Chem., 31, 2493 (1993).
These and other related molecularly-dispersed polymer/layered silicate nanocomposites have been shown to achieve a higher degree of stiffness, strength, heat and flame resistance, and barrier properties, with far less inorganic content than comparable glass (or mineral) reinforced “filled” polymers as a result of the molecular scale interaction between the two components.
Another example of a nanometer-scaled composite is a nanocomposite of nanoparticles and conjugated polymers, designed to achieve composition-tunable optical constants for use in semiconducting photonic structures. The 550-nm wavelength in-plane refractive index of poly(p-phenylenevinylene)-silica composites can, for example, be tailored over the range of 1.6 to 2.7 using this approach, allowing the fabrication of efficient distributed Bragg reflectors and waveguides. See, for instance, P. K. H. Ho, D. S. Thomas, R. H. Friend, N. Tessler, Science, 285, 233 (1999).
Inorganic nanoparticles dispersed in conducting polymers have also been formed into nanocomposites with potentially useful colloidal stability, optical, dielectric, magnetic susceptibility, electrochromic and catalytic properties. For example, see R. Gangopadhyay, A. De, Chem. Mater., 12, 608 (2000). H. J. Snaith, G. L. Whiting, B. Sun, N. C. Greenham, W. T. S. Huck, R. H. Friend, NanoLett., 5, 1653 (2005).
While the above examples are predominantly organic-inorganic nanocomposites, strictly inorganic nanocomposites are also expected to be of high utility. As for the organic-inorganic hybrids, useful properties can be expected as a result of the nanometer scale integration. Strictly inorganic nanocomposite are expected to be particularly desirable compared to organic-inorganic hybrids because of the improved thermal and mechanical stability of the composite, as well as the potential for much better electrical transport characteristics (e.g., carrier mobility).
In addition to obtaining inorganic nanocomposites with useful properties, it is desirable to be able to process the nanocomposites into useable forms using relatively low-cost and high throughput fabrication techniques. For electronic applications, thin films are commonly the targeted form and solution-based techniques such as spin coating, stamping, drop casting, doctor blading or printing are all ideal examples of potentially low-cost high-throughput processes. Unfortunately, most bulk inorganic semiconductors of interest are not soluble in common solvents. Recently, however, inventors of the present application have demonstrated a solution-processing technique for the deposition of high-mobility metal chalcogenide films using hydrazine or hydrazine/water mixtures as the solvent. For example, see U.S. Pat. No. 6,875,661, Mitzi et al. Nature, 428, 299 (2004) and Mitzi, J. Mater. Chem., 14, 2355 (2004).
The process involves forming soluble hydrazine-based precursors of targeted metal chalcogenide semiconductors (typically SnS2-xSex), spin coating films of the precursors, and thermally decomposing the precursor films at low temperature to form the desired continuous metal chalcogenide films. Additionally, n-type indium(III) selenide films have been spin-coated, using the corresponding hydrazinium precursor and mixed ethanolamine/DMSO solvents. See Mitzi et al., Adv. Mater., 17, 1285 (2005).
Thin-film field-effect transistors (TFTs) have been constructed, based on the solution-deposited semiconducting chalcogenides, yielding n-type channels with electron mobilities in excess of 10 cm2/V-s, approximately an order of magnitude better than previous results for spin-coated semiconductors.
Nanoparticles and other nanoentities also provide an exceptional degree of electronic and structural flexibility, primarily exemplified by the ability to continuously tailor the size of the particles and therefore, via quantum confinement effects, the electronic properties of the particles. The electronic and boundary properties of the particles can also be controlled by employing a core-shell approach for fabricating the nanoparticle wherein the core of the nanoparticle is a different material than the outer shell. See Cao et al., J. Am. Chem. Soc., 122, 9692 (2000). Additionally, nanoentities of different shape can be fabricated, ranging from spheres, to rods, wires and tetrapods. See Manna et al. J. Am. Chem. Soc., 122, 12700 (2000). Typical tetrapod-shaped nanocrystals contain at least four arms. The shape includes a nuclei with arms growing out of each of the four (111) equivalent faces. In some configurations, arms of the tetrapod have branches growing in the vicinity of their ends. Different functionalization of the nanoparticles can affect the dispersability of the nanoparticles in solvents and provides another degree of flexibility.